Method for producing mono-dispersed spherical granules

ABSTRACT

The present invention relates to a method that comprises dispersing a stream of a melt flowing from a die by applying perturbations to said stream in an inert cooling gas which has an optimal temperature and is depleted of oxygen up to a value not exceeding 0.0001 mol. %. after their output at a stationary generation mode, the granules are recovered in the outlet portion of a heat-exchange chamber. The die is made of a heat-resistant material and has a flow section with a length defined by the relation 2d&lt;1&lt;20d. The perturbation frequency of the stream is defined by relation 
     
       
           f=Wk   o   /πd   o (1+ cτ ) 2   (I) 
       
     
     where τ is the dispersion time (at the initial moment τ=0); c is the empirical coefficient characterizing the die material resistance to the perturbation of the stream; w is the flow rate of the stream; d o  is the initial value of the stream diameter; and k o  is equal to 0.7 and is the value of the non-dimensional wave number. The material to be dispersed consists of a chemically active melted metal or alloy that comprises at least one rare-earth element.

TECHNICAL FIELD

The present invention relates in general to powder metallurgy, morespecifically to methods for preparing monodisperse materials used inregenerative heat exchangers, and has particular reference to a methodsfor preparing monodisperse spherical granules.

BACKGROUND ART

Known in the present state of the art is a method for preparing metalgranules (SU, A, #532,472) by a forced capillary disintegration of astream of melt under the action of regular perturbations. A devicedisclosed in the aforementioned reference operates by the methodmentioned before. However, the method leaves out of account the thermalcharacteristics of the process which involves low quality of theresultant granules as to spherical shape and monodisperse naturethereof.

The closest to the proposed method is a method for preparingmonodisperse spherical granules (RU, A, 2,032,498) which is based on theeffect of forced capillary disintegration of a stream of melt under theaction of perturbation applied thereto. The drops resultant fromdispersion of said stream of melt are cooled, under optimum conditions,with an inert gas that fills the flight chamber. The prepared granulesare taken out in the outlet section of the heat-exchanging chamber afterthe process has reached steady-state operating conditions of dropgeneration. When the stream of a chemically active melt flows through adie the surface of the flow-through orifice thereof gets eroded, wherebythe die orifice diameter increases with time. This in turn results inthat the stream diameter increases incessantly and the diameter of dropsinto which the stream is disintegrated.

Furthermore, the method under discussion suffers from a low quality ofdispersed material obtained from dispersing chemically active melts towhich, particularly, can be related rare-earth metals and alloysthereof.

DISCLOSURE OF THE INVENTION

It is a principal object of the present invention to provide a methodfor preparing monodisperse spherical granules which makes possibleattaining higher quality of dispersed material resulting from dispersingchemically active melts so that the root-mean square (standard)deviation of the granule diameter from the preset value should be within2% and the ratio between the greater and lesser granule diameters bewithin 1.02.

The foregoing object is accomplished due to the fact that in a knownmethod for preparing monodisperse spherical granules, according to whichthe stream of melt outflowing from the die is dispersed under the effectof perturbations applied thereto at an optimum temperature of thecooling gas and the resultant granules are taken out in the outletsection of the heat-exchanging chamber after the process has reachedsteady-state operating conditions of drop generation, according to theinvention, the inert gas is freed from oxygen to a maximum content of0.0001 mol. %, the die is made of a refractory metal, and the length ‘l’of the die flow section is within the range of 2d<1<20d, while thestream perturbation frequency is selected from the relationship:

f=Wk _(o) /πd _(o)(1+cτ)²

where:

τ—is the dispersion time (equal to zero at the initial instant of time),

c—is the empirical coefficient characteristic of the die materialresistance to the effect of stream perturbation,

w—is the stream outflow velocity,

d_(o)—is the initial stream diameter value,

k_(o)—is the initial value (0.7) of the dimensionless wave number, usebeing made of a material subjected to dispersion comprising at least oneof the following rare-earth metals: Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd,Tb, Dy, Ho, Er, Tm, Yb.

In what follows the present invention will now be disclosed in adetailed description of an illustrative embodiment thereof withreference to the accompanying drawings, wherein:

FIG. 1 is a device that carries into effect the herein-proposed method,according to the invention;

FIG. 2 shows granules prepared from Er₃Ni at a constant melt streamperturbation frequency;

FIG. 3 shows granules prepared from Er₃Ni at a melt stream perturbationfrequency changed according to the proposed ratio.

BEST METHOD OF CARRYING OUT THE INVENTION

The device carrying into effect the herein-proposed method comprises aheatable crucible 1 and a die 2 fixed in place at the bottom thereof, astream perturbation unit 3, a melt pressure-applying unit 4, aheat-exchanging chamber 5, a coolant gas temperature regulator 6, agranule separator 7, a coolant gas purifier 8, and a granule sizemonitor 9.

The device operates as follows. The heat-exchanging chamber 5 and thegranule separator 7 are filled, through the coolant gas purifier 8, withan inert gas having the oxygen content not in excess of 0.0001 mol. %.The metal ingots to be dispersed are melted down. A streamlined flow ofthe resultant melt is established using the melt pressure-applying unit4. The stream of melt is exposed to the effect of perturbation for saidstream to disintegrate at the following frequency:

f=Wk _(o) /πd _(o)(1+cτ)²

where:

τ—is the dispersion time (equal to zero at the initial instant of time),

d_(o)—is the initial stream diameter value,

w—is the stream outflow velocity,

k_(o)—is the initial value (0.7) of the dimensionless wave number (cf.J. W. Rayleigh, “The Theory of Sound”, v.2) which is realized at theinitial period of the granulation process. Within the starting period ofthe device the resultant granules are collected in an auxiliarycontainer of the separator 7. Once the steady-state drop generationconditions have set in, the main container of the separator 7 is filledwith the granules obtained. The size of the resultant drops is monitoredusing the fiber-optic granule size monitor 9.

In the device realizing the proposed method for granulating chemicallyactive melts the heat-exchanging chamber 5 is filled with helium havingthe oxygen content not over 0.0001 mol. %. With a higher oxygen contentof helium the proposed granulation method is impracticable because astabilizing oxide film is formed on the stream surface which preventsstream disintegration into drops.

Reaction between the stream of a chemically active melt and the materialof the die 2 results inflicts erosion upon the orifice of the die 2. Itis common knowledge that there exist no materials absolutely resistantto the action of melts of rare-earth metals. It is refractory metals(molybdenum, tantalum, tungsten) that can be regarded as the materialsmost resistant to such action. However, even in the case of saidrefractory metals the material of the die 2 is subject to time-dependenterosion, whereby its orifice is increased by up to 50% for 30 min.

It is found experimentally that when a chemically active melt outflowsfrom the die 2, an optimum length of the die orifice is within the rangeof 2d<1<20d. The lower limit is defined by an abrupt rise of the rate oferosion of shorter die orifices, while the lower limit is concerned withthe fact that a velocity profile is formed on a stream outflowing from alonger die orifice which tells unfavorably on stability of the processof forced capillary disintegration of a stream of melt.

It is due to erosion of the flow section of the die 2 that the resultantgranules are polydispersed ones. Deterioration of the quality ofdisperse material concerned with a time-dependent increase of the streamdiameter can be eliminated by properly adjusting the operatingconditions of the device (i.e., the flow velocity and perturbationfrequency of a stream). With a higher stream perturbation frequency thediameter of drops gets time-stabilized at a preset level. Timedependence of a change in the stream perturbation frequency can beobtained from consideration of an equality between the volume of a dropand the length of stream from which said drop is formed:

πd ²/4×w/f=πd ³/6  (1)

where D is the drop diameter. From (1) we obtain:

f=3wd ²/2D ³.  (2)

As experience has shown, time-dependent changes in the diameter oforifice of the die 2 is well described by the linear relationship:

d/d _(o)=1+cτ  (3)

where:

d_(o) is the initial value of the die orifice diameter (τ=0), d is thevalue of said diameter at the time instant τ, c is the empiricalcoefficient characteristic of the resistance offered by the material ofthe die 2 to the action of the melt.

Hydrodynamic resistance of the die 2 is defined largely by a local flowconstriction resistance which is but little dependent on the orificediameter. Therefore the stream velocity may be assumed constant, with anerror on the order of 1% which is practically quite sufficient. Takingaccount of the above-said and using (2) and (2) a condition forregulating the stream perturbation frequency is derived, which, whenfulfilled, ensures constant diameter of the resultant drops:

f=f _(o)(1+cτ)²  (4)

where:

f_(o)=k_(o)w/πd_(o)—is the stream perturbation frequency at the initialinstant of time τ=0. Stream perturbation at the initial instant of timeis effected with the wave number k_(o)=0.7 which corresponds to therange of maximum stream instability [3].

It is noteworthy that monodispersing of a melt stream having atime-increased diameter under conditions of perturbation frequencycorrection may be carried out within a restricted period of time, thatis, until the dimensionless wave number ‘k’ exceeds unity. In the rangeof k>1 the stream gets hydrodynamically stable so that the effect offorced capillary disintegration of the stream on which is based thegranulating techniques proposed herein, is degenerated.

The data on the techniques of preparing a monodisperse material from thealloy of Er₃Ni used in regenerators of cryogenic gas machines aretabulated below. The table contains the following data: d_(o)—initialvalue of the orifice diameter in the die 2; d_(f)—finite value of saidorifice diameter; τ_(f)—duration of the granulating process; P—excesspressure in the crucible; w—stream velocity; f_(o)—initial streamperturbation frequency; c—empirical coefficient used for determiningstream perturbation frequency; x—concentration of oxygen in helium;T₁—melt temperature; T₂—coolant gas temperature; D—diameter of theresultant granules; δ₁—root-mean square (standard) deviation of thegranule diameter from the preset value; δ₂—maximum value of the ratiobetween the greater and lesser granule diameters.

TABLE x d_(o) D_(f) τ_(f) P w f_(o) C mol T₁ T₂ D δ₁ δ₂ μ μ s Mpa m/sl/c l/c % K K μ % % 80 104 1400 0.54 3.5 9800 2E−4 8E−5 1173 450 150 1.51.01

INDUSTRIAL APPLICABILITY

The present invention can find application for preparing monodispersematerial used in regenerative heat exchangers.

What is claimed is:
 1. A method for the preparation of monodispersespherical granules comprising at least one rare-earth metal selectedfrom the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm and Yb comprising: melting the metal to be prepared asspherical granules in a crucible to form a melt; establishing aperturbation in the melt; forcing the melt under a variable head ofpressure through a die at the base of the crucible into aheat-exchanging chamber, said die being formed from a refractory metaland having a length between 2 and 20 times its diameter whereby dropletsare formed by the melt exiting the die; cooling said heat-exchangingchamber with a cooled, purified inert gas having an oxygen content lessthan or equal to 1.0×10−4 mol. %; monitoring the size of the droplets inthe chamber; and collecting the granules at the bottom of the chamber;wherein the stream perturbation frequency f defined by equation (1):f=Wk _(o) /πd _(o)(1+cτ)²  (1) where: τ—is the dispersion time (equal tozero at the initial instant of time), c—is the empirical coefficientcharacteristic of the die material resistance to the effect of streamperturbation, w—is the stream outflow velocity, d_(o) is the initialstream diameter value, k_(o)—is the initial value (0.7) of thedimensionless wave number is adjusted to maintain monodisperse sphericalgranules in response to the monitored size of the droplet.